In a conventional suspension system for a steerable wheel, the knuckle which rotatably supports a wheel is typically pivoted at its upper and lower points, respectively, to the free ends of, for instance, a pair of A-arms which are pivotally connected to the vehicle body at their base ends so as to allow a vertical motion and a steering action of the wheel. In such a suspension system, an imaginary kingpin can be defined as a line connecting the upper and the lower pivot points of the knuckle. The wheel is steered about this imaginary kingpin but, since the center of the tread contact surface of the wheel and the point of intersection between the imaginary kingpin and the tread contact surface generally do not coincide with each other, the so-called self aligning torque which tends to restore the wheel to a straight ahead position is normally produced.
When a manual steering system is used, this self-aligning torque is naturally transmitted to the steering wheel and gives the feel of the state of the steered wheels to the driver. Even when a power assisted steering system is used, the reaction which the wheels experience from the road is allowed to be transmitted from the steered wheels to the steering wheel so as to improve the handling of the vehicle by giving the feel of the steered wheels to the driver. In either case, this self-aligning torque is required to be controlled to a manageable level and should not increase or decrease abruptly as the steering angle is changed.
In the case of steerable rear wheels of a vehicle equipped with a four wheel steering system in which the ratio of the steering angle of the rear wheels to the steering angle of the front wheels is varied depending on the speed of the vehicle or the steering angle of the front wheels, the self-aligning torque may change in a highly complex way without offering any benefit and it is desirable to totally eliminate the self-aligning torque if possible.
When the kingpin, real or imaginary, is fixed as is the case with most of the conventional suspension systems, the self-aligning-torque can be easily eliminated by allowing the kingpin to pass through the center of the tread contact surface of the wheel but when that cannot be done because of other considerations then it is impossible to control the self-aligning torque.
On the other hand, in a suspension system of the kind disclosed in Japanese Patent Publication No. 52-9889 having two pairs of lateral links connected to upper and lower parts of the knuckle, respectively, it is possible to change the various parameters of the suspension system and favorable settings of the camber angle, the toe angle and caster angle can be obtained as desired, but, since the imaginary kingpin tends to move as the steering angle changes, the resulting change in the self-aligning torque or the force required to maintain a steering angle tends to vary a great deal or in a complicated fashion and may not be sufficiently manageable. A favorable arrangement of such lateral links is proposed in copending U.S. Pat. Application No. 135,250 by the same Inventor and assigned to the same Assignee.
However, it is not always possible to achieve the above-mentioned geometrical relationship due to the restrictions in the freedom of layout of the suspension system. Specifically, since it is generally necessary to support the wheel against a running resistance with a certain compliance, one of the lower links is required to be constructed as a radius rod which forms a relatively large angle in relation with the direction of the axle. Further, due to the fact that the lower end of the shock absorber system comprising an oil damper, a coil spring and so on is desired to be supported by one of the lower links, it is preferable if the other lower link extends along and in the proximity of the axle as projected upon a horizontal plane. Generally speaking, the instantaneous centers of rotation of the lower links and the upper links can be made to coincide with each other as projected onto a horizontal plane by arranging the upper links and the lower links in a similar layout and reducing the range of the change of the instantaneous center of rotation at the tire tread surface. However, such a layout of the upper links is not possible in this case since the upper end of the shock absorber system tends to interfere with the upper links.